1. Field of the Invention
The present invention relates to a micro mirror unit in which a micro mirror can be moved electrostatically, an optical disc drive using the micro mirror unit, and a method for producing the micro mirror unit.
2. Description of the Related Art
In the recent electro-mechanical field of industry, the techniques in the semiconductor producing processes, etc. are utilized to actively develop techniques called “MEMS (micro electro-mechanical system)” intended for providing a variety of micro mechanical elements. A mirror designed with such MEMS techniques to be extremely compact as a light reflecting mechanical element is called “micro mirror unit”.
The micro mirror unit consists of a frame and a mirror body having a reflective surface. In the unit, the frame and mirror body are linked to each other by means of at least one hinge. The mirror body includes a portion formed from an aluminum (Al) layer for example and working as an electrode. Also, there is provided a pair of electrodes formed from an aluminum (Al) layer and opposite to the electrode-functional portion of the mirror body with an air gap between them.
In the micro mirror unit, voltages of different signs are alternately applied to the pair of aluminum-layer electrodes for developing opposite-directional electrostatic forces between the pair of electrodes and the electrode-functional portion of the mirror body to pivot the mirror body about the hinge to a tilted position in a selected direction.
In the above micro mirror unit, the mirror body and hinge are formed from a metal layer such as Al layer formed on a substrate as known from the disclosure in the Japanese Unexamined Patent Publication No. 7-287177 for example. More particularly, in the micro mirror unit, the surface of the metal layer formed on the substrate works as the mirror surface of the mirror body, and the hinge is formed from a similar metal layer to that of the mirror body.
In the micro mirror unit constructed as in the above, the mirror body and hinge are formed from a single metallic material, so that the micro mirror unit is advantageously easy to produce. However, since the micro mirror unit is adapted for the mirror body thereof to be pivotable about the hinge to a tilted position in a selected direction, the hinge will possibly be damaged due to a so-called metal fatigue as the mirror body is repeatedly pivoted.
To avoid the above problem, a micro mirror unit has been proposed in which a mirror substrate and hinge, being main parts of the mirror body, are built in an Si substrate material forming a frame and a metal layer is formed on the mirror substrate to provide a mirror surface (as in “Technical Digest of the 16th Sensor Symposium”, 1998, pp. 167-170). Namely, in this micro mirror unit, the mirror substrate, hinge and frame are formed integrally with each other by a micro working of the single Si substrate material by the wet etching. Further in this micro mirror unit, the mirror substrate made of Si has a highly-doped layer formed therein for the mirror body to be electrostatically movable.
In the micro mirror unit in which the mirror substrate and hinge are built in the Si substrate material forming the frame, the hinge has a higher resistance against the pivoting of the mirror body than a one made of a metal and thus is not so easily damaged.
In the micro mirror unit constructed as in the above, however, since the hinge is formed from the Si substrate material forming the frame as in the above, the shape of the hinge is limited to the thickness of the Si substrate material. Also, since the micro working by the wet etching takes a very long time, the micro mirror unit having the frame, mirror substrate and hinge formed by the wet etching will be produced with a poor efficiency and with an increased cost.
Further, in the micro mirror unit, since the hinge is formed by micro working of the single Si substrate material by the wet etching, it cannot easily be formed more finely and with a high precision. That is, in case the hinge is formed by the wet etching, the hinge having desired micro dimensions cannot be formed with a high precision because of the limited precision of the wet etching in the micro working.
Recently, techniques for using a micro mirror unit as a micro-motion actuator in an optical disc drive have been proposed. For a micro mirror unit suitable for use as such a micro-motion actuator in the optical disc drive, the hinge thereof will be required to be formed to have a further micro structure. In the micro mirror unit, the mirror body is twisted and oscillated about the hinge when being pivoted as having been described in the above. Therefore, the accurate motion of the mirror body greatly depends upon the shape of the hinge, and the hinge should be worked more precisely to a desired shape so that the micro mirror unit can work accurately as a micro-motion actuator.
Accordingly, there has been demanded a micro mirror unit whose hinge can be formed further finely and with a high precision and having a novel structure suitable for use as a micro-motion actuation in an optical disc drive.